This disclosure relates to a variable displacement pump, and more particularly relates to a rotating cam ring vane pump that employs a fluid bearing to support the cam ring.
Current rotating ring vane pumps use a cam ring fluid or journal bearing fed with high pressure from the pumping element. This journal bearing acts as a combination hydrostatic and hydrodynamic bearing. The cam ring that is supported by the bearing is driven by friction between the cam ring at the interface with the vanes. At low speeds, typically on the order of approximately twenty percent (20%) of maximum speed or less, the friction generated between the vanes and cam ring is not high enough to start rotation of the cam ring. When the cam ring is not rotating, mechanical efficiency is reduced. This, coupled with the reduced volumetric efficiency due to leakage through the cam bearing ring, will result in the pumping element sizing point at a low speed condition, typically on the order of less than ten percent (10%) of maximum speed.
The physical size and weight of the pump are important to the system design. It is desirable to minimize the pump flow capacity in order to minimize physical size and weight. In many fuel systems that incorporate positive displacement type pumps, pump flow capacity is set either at engine take-off conditions or at engine start conditions. Sizing the pump flow capacity at the take-off condition minimizes physical size and weight of the unit. Sizing at engine start conditions is typically an outcome of the level of parasitic internal leakage of the fuel system.
As fuel system parasitic internal leakage is a controllable quantity by specific system design, the minimization of that leakage will result in a pump sized at the more desirable take-off condition. Cam bearing flow forms part of the fuel system parasitic leakage quantity. Therefore, elimination of the cam bearing flow at low speeds, such as windmill engine start, helps achieve pump sizing at the take-off condition. Curtailing of cam bearing flow leads to higher pump flow capacity at specific operating conditions; therefore, the engine start condition is provided as a representative condition for curtailing cam ring bearing flow and could be performed at any condition in which extra pump flow capacity is desired.
While curtailing cam ring bearing flow in effect increases the pumping system volumetric efficiency, it can be accompanied by a significant loss in mechanical efficiency. Further, the gain in volumetric efficiency can be outweighed by the loss in mechanical efficiency and thus results in a lower overall pump efficiency. For this reason, the selectable application of cam ring bearing flow is desired.
In an ever increasing need to improve efficiency, manufacturers are seeking to reduce the weight of individual components where ever possible. Selectable application of cam ring bearing flow leads to optimization of pump performance over the wide range of operating conditions typically encountered by a variable displacement device. Accordingly, re-design of the system and operation of the fuel pump can result in significant savings.